The communications technology used for non-contact-type IC cards such as FeliCa supplies power and exchanges signals by use of a card reader and writer that generates an alternating current magnetic field, whose changes are converted by an antenna coil in the IC card into an induced current. During a short time in which the IC card is held over the card reader and writer, the power required by the IC card to operate is transmitted to the IC card, which then conducts data communication based on the received power. This FeliCa communications technology is already implemented in some portable phones, and is used in transactions such as ones for making a payment. Portable apparatuses such as portable phones are provided with an internal battery, so that no power is fed from a card reader and writer during communication.
In portable apparatuses such as portable phones, a contact-type power feeding method is currently employed in order to charge the internal batteries. It is expected, however, that in near future, a non-contact type power feeding method using magnetism such as electromagnetic induction or magnetic-field resonance will be used in portable apparatuses. It might be possible to transfer power not only over a short distance but also over a relatively long distance by use of magnetic-field resonance. In such a case, placing a portable apparatus on a desk or the like in a casual manner would be sufficient to easily charge the apparatus, which improve usability. The principle of the non-contact power feeding is basically the same as the principle of power transmission used in FeliCa. Changes in the magnetic field generated at the power transmission side are converted into an induced current by a coil at the power receiving end, thereby transmitting power.
In portable phones provided with FeliCa, a magnetic shield is disposed between an antenna coil and the core of the portable phone. The antenna coil is disposed near the exterior surface of the outer case of the portable phone while circuits constituting the portable phone's internals inside the case are comprised of materials inclusive of metal such as a circuit board and a battery. In the absence of the magnetic shield, magnetic flux generated by the transmission side and passing through the antenna coil at the receiving end generates eddy currents in the metallic materials of the internals. Such eddy currents generate a reactive magnetic field, and also result in power loss caused by eddy current loss. This gives rise to problems such as communication failure and a drop in power feeding efficiency.
In consideration of the above, a magnetic shield is placed behind the antenna coil disposed near the surface, i.e., placed between the antenna coil and the internal circuits. This magnetic shield is composed of a material having a large permeability (i.e., the real part of permeability) and a small magnetic loss (i.e., the imaginary part of permeability), so that the magnetic field forms a circulating flow along the magnetic shield to return to the transmission side. Accordingly, the magnetic field does not reach the metallic materials inside the portable phone, so that there is neither a reactive magnetic field nor electric loss generated by eddy currents.
In order to cause the magnetic field to form a circulating flow, the product of the volume and permeability of the magnetic shield may need to be no smaller than a predetermined value. Further, low magnetic loss may be needed to reduce the loss of the magnetic field inside the magnetic shield. In general, the magnetic property of magnetic material is frequency dependent. Because of this, high permeability and low magnetic loss may need to be present at 13.56 MHz that is the frequency used by FeliCa.
When power is transmitted to a portable phone through non-contact power feeding as previously described, the use of frequency coincident with the frequency used by FeliCa gives rise to the problem of mutual interference, and may also end up destroying circuits used for FeliCa. While the amount of transmitted power in FeliCa is in the order of micro watts, power transmission for charging purposes may require as large power as several watts. There is a difference by several orders of magnitude. It is possible to provide newly manufactured apparatuses with a mechanism to switch between the power transmission system and the FeliCa system. However, existing FeliCa apparatuses are not provided with such a protective mechanism. It is thus not feasible to use the frequency coincident with the frequency used by FeliCa. Further, it is difficult to create an amplifier that generates an alternating current magnetic field at high frequency. In general, a frequency lower than 13.56 MHz that is the frequency used by FeliCa may preferably be used for non-contact power feeding
In portable apparatuses such as portable phones, a large number of functions are implemented within a limited space, so that space conservation is extremely important. When both the FeliCa function and the non-contact power feeding function are implemented, thus, the antenna coil for FeliCa and the antenna coil for non-contact power feeding are preferably arranged in an overlapping manner at the same place, rather than being arranged at different places. Alternatively, the antenna coil for FeliCa and the antenna coil for non-contact power feeding may be implemented as a single shared coil. In this case, however, the frequency characteristics of the magnetic shield become a problem.
The magnetic shield disposed behind the coil for the FeliCa and non-contact power feeding purposes may need to have high permeability and low magnetic loss at both the frequency used by FeliCa and the frequency used by the non-contact power feeding. Generally, however, no magnetic shield exists that has frequency characteristics exhibiting high permeability and low magnetic loss over all frequencies. What causes magnetism is a magnetic moment created by rotational movement of en electron. When a low-frequency alternating-current magnetic field is applied to a magnetic material, the movement of magnetic moments follows the changes of the magnetic field, thereby providing high permeability (i.e., the real part thereof). As the frequency of the alternating current magnetic field applied to the magnetic material is increased, the magnetic moments gradually fail to follow the changes of the magnetic field, resulting in a decrease in permeability (i.e., the real part thereof) and an increase in magnetic loss (i.e., the imaginary part of permeability). When the frequency is further increased, the magnetic moments stop moving by completely failing to follow the high frequency, resulting in both permeability and magnetic loss being decreased. Accordingly, a magnetic material exhibiting high permeability at high frequency is hard to exist. It is difficult to develop an ideal magnetic shield that has high permeability over all frequency ranges.
The permeability of magnetic material decreases as frequency increases above the magnetic resonance frequency. A relationship (i.e., Snoek's limit) exists between the magnetic resonance frequency and permeability, i.e., the lower the permeability, the higher the magnetic resonance frequency is. In the case of a magnetic shield having a relatively high permeability, thus, the magnetic resonance frequency is low, and permeability starts to drop with an increase in magnetic loss in a relatively low frequency range. In the case of a magnetic shield having a relatively low permeability, the magnetic resonance frequency is high, and permeability starts to drop with an increase in magnetic loss in a relatively high frequency range.
A magnetic shield used for non-contact power feeding in low frequencies may be required to have an extremely high permeability in order to cause a strong magnetic field for non-contact power feeding to be fully circulated. Such a magnetic shield having high permeability, however, has a low magnetic resonance frequency, which means that permeability in a high frequency range used by FeliCa or the like is significantly lowered. Conversely, an attempt to secure a reasonable permeability in high frequencies used by FeliCa or the like ends up employing a magnetic shield having a high magnetic resonance frequency. In this case, a sufficiently high permeability that can completely circulate the strong magnetic field for non-contact power feeding is not provided.
Patent Document 1 discloses a configuration in which a power-purpose coil unit and a signal-purpose coil unit are arranged side by side, and two magnetic shield plates are disposed between the power-purpose coil unit and the signal-purpose coil unit. In this configuration, there is a risk of undermining accurate signal transmission when a magnetic field leaking from the power-purpose coil unit interferes with magnetic signals generated by the signal-purpose coil unit. Because of this, the two magnetic shield plates are provided between the power-purpose coil unit and the signal-purpose coil unit to remove or alleviate the effect of leaked magnetic flux. One of the magnetic shield plates is made of a conductive material having large eddy current loss, i.e., having low electrical resistivity. The other one of the magnetic shield plates is made of a high-permeability material effectively absorbing magnetic flux, such as soft magnetic material like Mn—Zn ferrite. These two magnetic shield plates are for the purpose of reducing the effect of noise made by the power-purpose coil unit on the signal-purpose coil unit, and are not for the purpose of providing a proper shield in different frequencies that are used for the power purpose and for the signal purpose, respectively. Further, the magnetic shield plates made of conductive material are not fit for actual use because its eddy current loss is undesirably large in high frequencies that are supposed to be used in non-contact power feeding used at present.